Pharmacological manipulation of neurotransmitter metabolism has been a powerful tool in the treatment of nervous and mental diseases. Many advances in psychopharmacology have resulted from our basic knowledge of neurotransmitter synthesis and degradation. The broad objective of the proposed research is to elucidate the metabolic control mechanisms which regulate the synthesis of one neurotransmitter, gamma-aminobutyric acid (GABA). The present evidence indicates that glutamate decarboxylase (GAD), the enzyme responsible for GABA synthesis, may be regulated in part by an interaction between the pyridoxal-5-phosphate (the cofactor required by the enzyme) and the adenine nucleotides ATP and ADP. This concept is based on kinetic studies with crude preparations of the enzyme and are supported by the observation of a post-mortem increase in the degree of saturation of GAD by pyridoxal-P in the intact animal. Other evidence indicates that glutamate decarboxylase exists in both high- and low-molecular weight forms. In addition, it has been suggested that GABA feeds back to inhibit pyridoxal kinase, the enzyme responsible for the synthesis of pyridoxal-P. The proposed studies are designed to attack this problem from two directions: first, by detailed studies of the purified enzymes, and second, by studies of the regulation of GAD in brain slices and in the intact animal. In the case of GAD, studies will include purification of both high- and low-molecular weight forms of the enzyme and detailed studies of the binding of pyridoxal-P, ATP, ADP and Pi to the enzymes. In addition, the kinetic properties and subunit structures of the two forms of the enzymes will be compared. Studies of pyroxidal kinase will deal primarily with the mechanism of inhibition of the enzyme by GABA in an effort to determine whether this inhibition by GABA can take place in vivo. Studies in brain slices and intact animals will be designed to determine if changes in energy metabolism are reflected in changes in the degree of saturation of GAD. This will be done by perturbing energy metabolism with treatments known to affect brain GABA levels.